The process for cleaning grain in combine harvesters has not changed fundamentally for many decades. The cleaning unit, or ‘shoe’, has directed therethrough a cleaning airstream which is typically generated by a cross-flow or centrifugal fan located in front of the cleaning shoe. As a mix of grain kernels, chaff, tailings and straw is passed over one or more oscillating sieves, the cleaning airstream serves to blow the lighter material in a generally rearward direction over the sieves and out of the rear of the machine. The grain is generally heavier and/or smaller than the material other than grain (MOG) in the mix and passes through the sieves.
The cleaning shoe is most efficient when the grain is caused to settle on the uppermost sieve (hereinafter referred to as the chaffer) as early as possible and the lighter material is kept airborne. If the grain bounces on impact with the chaffer then the risk of the cleaning airstream carrying the grain out of the rear of the shoe increases. The speed of the cleaning airstream is typically selected to strike a balance between grain cleanliness and acceptable loss. Ultimately, this balance creates a limit on the capacity of the cleaning unit. In other words, without an increase in shoe size, any increase in capacity will adversely affect grain cleanliness and/or loss rate.
However, despite these limits, there remains a continuing drive to increase the size and capacity of combine harvesters to meet the needs of modern farmers and to speed up the overall harvesting process. As outlined above, increasing the throughput of the cleaning shoe with known technology requires an increase in the physical size. For example, increasing the width of the cleaning shoe would deliver an increased machine capacity but at the cost of increased machine width. However, maximum machine width is limited by road transport legislation in some countries thus rendering this option unattractive. In a similar vein, increasing the length of the chaffer would require an increase in wheel base and a consequential increase in turn radius which is undesirable to farmers.
Efforts to increase the capacity of the cleaning shoe based on pre-stratification of the crop stream have been made. For example, WO-2012/095239 discloses a combine harvester having an extended return pan which catches separated crop material from overhead threshing and separating apparatus and conveys such to a front edge from where it drops on to a grain pan, the grain pan conveying the crop mix rearwardly to a rear edge from where it falls into the cleaning shoe. In this disclosure, the return pan is of an extended construction to deliver the majority of the separated material to the front of the grain pan to facilitate enhanced stratification of the material before delivery to the cleaning shoe. As disclosed therein, the recognised advantage that the grain rich bottom layer falls directly onto the chaffer whereas the upper MOG-rich layer is rendered airborne by the cleaning airstream.
The process of stratification has been found to be very sensitive to working on side-hills. It is known to provide structure on the grain pan in the form of longitudinal fins which prevent the crop material from piling up on one side of the grain pan when operating on a side-hill. However, even the provision of such fins does not aid stratification of the grain and MOG.